Meddle With the Pedal: ELECTRONIC THROTTLE CONTROL

Motor, Jul 2007 by Dale, Mike

Doing away with the throttle cable was just the beginning. Electronic throttle control (ETC) has allowed engineers to add many other noteworthy vehicle systems and capabilities, with more to follow.

It may well be that software integration of automotive electronic systems will turn out to be the most significant automotive technological development of this decade. Originally electronic systems such as ABS, HVAC and emissions were developed separately by those groups within each carmaker that were most responsible. The brakes and suspension group worked on ABS while emissions and engine control issues were handled by powertrain people. Software integration has brought these systems together. The result is new, interrelated technologies that produce better mileage, safer cars and reduced emissons.

At the head of this trend, as an enabling technology, is electronic throttle control (ETC), which is part of an industrywide response to calls for better fuel economy, reduced emissions and a reduction in vehicular fatalities. This story is not so much about hardware as it is about software that uses ETC as an input and an actuator to make the new technologies possible.

Without ETC, the planned advances in hybrid and diesel technology that are now right around the corner would not be possible. Current advances such as electronic stability control (ESC), expected to save thousands of lives per year, would simply not be possible without ETC. Best yet, ETC reduces cost and complexity for carmakers by integrating formerly stand-alone features such as idle control, cruise control and throttle control into a single, mostly software-based system.

This latest version of electronic throttle control should not be confused with the earlier stand-alone systems that replaced the mechanical link between the driver and the engine. In these new systems, the output of the pedal sensor is an input not only to the engine control system but to the software system as a whole. As such, pedal angle becomes a valuable input to other electronic control systems. The algorithms' that control the ABS, ESC, cruise control, HVAC and other system functions all use pedal angle data in the decision-making process. The throttle angle that results is not only what the driver wants but what the systems needs for correct and safe operation.

In these new-generation ETC systems, the accelerator pedal module becomes a two-way device: It, accepts information about desired engine output from the driver, plus it can feed back tactile information to the driver as a warning that the selected engine output is either wrong or dangerous.

The Technological Need for ETC

The clear goals of the automotive industry are to improve fuel economy, reduce emissions and improve function and safety for the driver. To understand the design options available to accomplish these goals, you need to know what produces the best results and what causes subpar performance. These complicated goals are further complicated by trade-offs that have to be made.

Fuel economy and emissions output per mile traveled are directly related to the size of the vehicle and the size of the engine. In keeping with the laws of physics, cutting fuel consumption is about either reducing the mass of the vehicle or reducing acceleration. Since the systems are not perfect, there's another path that can be traveled-by improving efficiency to reduce losses.

The first thing to know is that most automobile engines are much larger than they need to be for most real-world operating conditions. The big V8 often selected for full-size pickups is really chosen to pull a boat or trailer the owner may have in mind. Yet trailer towing may amount to less than 10% of the actual vehicle miles; 90% of the time a smaller engine would do just fine.

The fact that engines generally spend most of the time running at a small fraction of their peak power output is referred to as the partial power problem. Toyota says the Otto cycle engine is most efficient at 40% to 45% of its redline rpm. This is the point at which torque is at about 70% to 80% of its peak value for a given engine. In this most efficient operating range, the engine produces about 40% of its peak power rating.

Let's use Toyota's 108-hp ECHO engine as an example. Given the numbers just mentioned, it would be best if most of the time the engine output were in the range of 40 to 50 hp. Unfortunately, this is not enough for adequate acceleration or hill climbing. Calculations show that if the ECHO had only a 30-hp engine, it would need 30 seconds to accelerate to 60 mph. If such a vehicle were to encounter a 10% grade, it would slow down to 30 mph before it reached the top of the hill.

On the other hand, only 15 hp or so is needed to maintain 60 mph on level roads, and even less power is needed for idling and low-speed travel. The net result is that the engine power output that was chosen for adequate passing and hill-climbing is larger than necessary for most of the operating circumstances of the vehicle.